A commonly and widely used magnetic recording medium uses as a binder a thermoplastic resin such as a vinyl chloride and vinyl acetate type resin, a vinyl chloride and vinylidene chloride type resin, a cellulose type resin, an acetal type resin, a urethane resin, or an acrylonitrile butadiene resin alone or in combination. However, this type of magnetic recording medium has defects such as that wear resistance of a magnetic layer is so poor that a tape of a magnetic tape stains.
Further, it is known that a thermosetting resin such as a melamine resin or a urea resin is used as a binder or that a binder which is crosslinked by a chemical reaction such as an isocyanate compound or a compound having an epoxy ring is added to the above-described thermoplastic resin. However, the use of the crosslinkable binder is accompanied by defects such as that (1) a resin solution having magnetic particles dispersed therein has poor storage stability, and physical properties of a magnetic coating composition are not uniform and thus a magnetic recording medium cannot maintain its uniform quality, and that (2) a heat treatment for hardening a coated layer which is coated and dried is indispensable, and for the heat treatment it takes a long term.
In order to overcome the above defects, it is disclosed in Japanese Patent Publication No. 12423/72, Japanese Patent Application (OPI) Nos. 13639/72 (the term "OPI" as used herein means an "unexamined published application"), 15104/72, 77433/75, and 25231/81 that a magnetic recording medium is prepared by using an oligomer and a monomer of an acrylic acid ester type and hardening it by radiation exposure after it is dried. However, a magnetic recording medium having high electromagnetic properties and running durability cannot be obtained by the above mention.
Recently, a magnetic recording medium has been required to have higher image quality. In order to realize high image quality, it is necessary that a surface of a magnetic layer is more closely contacted with a video head and an audio head, that a surface smoothness of a magnetic recording medium is improved as well as dispersibility of ferromagnetic particles. On the other hand, the smoother the surface of a magnetic layer is, the higher is the friction coefficient of a running system using a video tape recorder, and running tension becomes higher. Therefore, a magnetic recording medium is required to have higher running durability under harsh conditions. In this connection, hitherto, a magnetic recording medium which completely satisfies all requirements as to surface smoothness of a magnetic layer, dispersibility of ferromagnetic particles and running durability of ferromagnetic particles, and running durability has not been produced in accordance with conventional methods.
In order to overcome the above-described defects, the inventors of the present invention have made extensive research as to a method of using a thermoplastic resin and a thermosetting resin, adding a binder crosslinkable with a chemical reaction thereto, and using a binder hardenable by crosslinking with radiation, and have finally attained the present invention.